In Photoshop there is a very easy way to build your own brushes but before you start there are some key things you need to know.

1. a solid black (#000000) is the equivilant of 100% opacity, a solid white (#FFFFFF) is the equivilant of 0% opacity
2. the larger you make your brushes the better it will be for larger image sizes. An example of this is if you take a small picture and stretch it out there will be large and blurry pixels, the same thing will happen with a brush because it is not vector based like a shape.
3. might as well save your brushes after creating them, you can always go back and delete them later.

now for the fun part where we actually start creating brushes.

1. open a new canvas 500x500px 300dpi
2. select a black on your color picker
3. drop your opacity
4. start paining
5. Select your marquee tool or 'M' on your keyboard
6. make a selection around what you have painted
(if you only painted one thing on your canvas it is easier to use CTRL+A to select all or Command+A if your on a mac)

7. Go to the top toolbar click: Edit > Define Brush Preset.

8. Give your brush a name and press 'Ok'
(when you have your brush tool selected you can right click on the canvas and try out your new brush, by default it will be in the last spot on the list)

Now that you have successfully created a brush there is a preset manager built into photoshop that can be used to manage your brushes: Edit > Preset Manager...

Once you are inside of the Preset Manager window there are multiple things you can do such as:
1. Move your brushes around for an easier access when working in Photoshop
2. Load brushes that you have downloaded
3. Save brushes. (CTRL or Command click multiple items to save them in a set)
4. Rename brushes that your not happy with.
5. Delete unwanted brushes

you dont have to follow this exactly when you make a brush and adobe is very flexible this way. It is possible to open a full color image, select the entire image and make a brush out of it. However, if you do this it will not be a full color brush, photoshop will convert it into black and white.

If you would like to download and try out brushes that i have created click here